It is known from the state of the art that a plurality of measuring apparatuses, which are arranged distributed in space, can be connected via measuring lines to a central controller. Each of the measuring apparatuses has at least one measuring element, with which a measured value is measured. This measured value is transmitted to the controller in analog form, i.e., as an analog current or voltage signal, because analog transmissions are often more robust than digital ones. To poll or set additional parameters of the measuring apparatuses at the same time, the controller can communicate via the measuring line digitally with such measuring apparatuses, which are set up for such digital communication and which are, in addition, connected to a terminal of the controller, which terminal is designed for this. The controller sends for this a digital message to a communication element of the measuring apparatus, which element can, in turn, respond to the digital message of the controller with a digital message of its own. For example, a higher-frequency signal, for example, at 1,200 Hz or 2,200 Hz, can be modulated for this onto the analog current or voltage signal, with which a measured value is transmitted from the measuring apparatus to the controller. The digital messages may be used, for example, to poll a state of a measuring apparatus or to perform a calibration of the measuring apparatus.
One example of such a system comprising a plurality of measuring apparatuses and a central controller is a distributed gas-measuring unit, in which a plurality of gas-measuring sensors are arranged distributed as measuring apparatuses over a production plant. The gas-measuring sensors measure, for example, a concentration of an explosive gas or of a gas that is hazardous to health in the ambient air and report the measured concentrations to the central controller. The HART protocol is an example of a communication protocol, according to which the communication can be handled between the controller and the measuring apparatuses.
The communication via the HART protocol or the digital communication via the analog measuring line may, however, also be used to maintain or even calibrate the measuring apparatuses. Portable data processing devices, such as laptops or mobile telephones, are regularly equipped for this with special hardware, which makes it possible to directly connect the data processing device to a measuring apparatus via a line. For example, the connection between the measuring apparatus and the central controller is cut first for this, and the measuring apparatus is connected, instead, directly to the data processing device. However, this requires a large amount of time and great technical effort. On the one hand, expensive special hardware is needed, which can communicate via the HART protocol, and, on the other hand, a maintenance technician must have access to each measuring apparatus to establish a physical connection to the measuring apparatus and to carry out the maintenance and calibration on site.
In addition, the measuring apparatuses can be connected to the data processing device via the controller, so that it is no longer necessary for the maintenance technician to have at least direct access to the measuring lines running to the measuring apparatuses. However, this procedure has the drawback that either the entire measuring system must be put out of operation for the maintenance or calibration and safety-relevant functions may possibly be deactivated during the time of the maintenance or calibration of individual measuring apparatuses, or at least some terminals of the controller and the measuring apparatuses connected thereto must be deactivated, so that at least these measuring apparatuses will now be out of operation.